In less than two years, I've restrung 7 of my 11 day/night shades, some more than once. They aren't difficult to restring, but it takes about an hour for the large four-string shades, counting time to remove and replace (if you are experienced).
Hints:
Moving both sections of a shade at the same time and moving the shade fast increase wear. When buying new string, look for smoothe string vs coarsely braided string because the latter wears faster. Make sure the strings are only tight enough to hold the shade up, as more tension increases wear.